In the field of data communications, a modem is used to convey information from one location to another. Digital Subscriber Line (DSL) technology now enables modems or other devices to communicate rapidly large amounts of data. Devices communicate by modulating a baseband signal carrying digital data, converting the modulated digital data signal to an analog signal, and transmitting the analog signal over a conventional copper wire pair using techniques that are known in the art. These known techniques include mapping the information to be transmitted into a signal space constellation, differentially encoding the information to reduce errors and improve throughput, and transmitting the information over a communication channel. The constellation can include both analog and digital information or often merely digital information.
At a receiver, the signal is received, equalized and decoded in accordance with techniques that those skilled in the art will appreciate.
In the above mentioned communications system, a control device, i.e., one that is located at a telephone company central office, connects on the communication channel to a plurality of remote devices typically located at a customer residential or business location. This topography is known as a multipoint communication environment because one control device is communicating with a plurality of remote devices co-located at a remote location, over a single communication channel. The communication technique between the control device and the remote device is generally half duplex in nature, meaning that only one device may transmit at any particular time. In order to establish a communications connection between the central office device and any of the remote devices, a preamble, which allows the devices to synchronize at the start of each message is required. Existing techniques such as carrierless amplitude/phase modulation (CAP) and discrete multitone (DMT) modulation allow modems to transmit simultaneously between only two devices at a time. In a multipoint environment, greater circuit efficiency is possible because of the ability to connect multiple devices to the same communication channel. In the above mentioned communication environment, noise is an everpresent obstacle to optimal receiver performance. Noise imparted by the communication channel can be substantially eliminated through the use of well known techniques such as precoding and channel equalization. Local noise imparted to a receiver, such as periodic impulse noise from local low frequency sources like light dimmers, and crosstalk from other signal sources presents a greater problem that can degrade receiver performance. All DSL equipment is susceptible to these impairments. In order to combat crosstalk, carrierless amplitude/phase (CAP) modulation uses a precoder while discrete multi-tone (DMT) selectively disables the affected frequency bins. The 60 Hz periodic impluse noise (such as that generated by a local electrical appliance such as a light dimmer) is allowed to exist and the resulting errors are corrected by a Reed Solomon (RS) code, which can delay the data. For multi-point applications however, the RS and DMT codes are ineffective. For example, the precoder used with CAP is located in the transmitter, and will cause discontinuities when the transmitter is switched on and off, as frequently occurs in a multipoint environment. Furthermore, the control device in the multipoint environment can have but one set of precoder coefficients for broadcasting to all remote devices, which may have different precoder needs. Also, ringing caused by on/off polling cannot be canceled by a precoder because the transmitter and precoder are inoperable during the ringing transient.
Therefore, it would be desirable to provide a noise suppression system at a communication device receiver to reduce or eliminate the local low frequency periodic impulse noise imparted to the receiver, while operating in a multipoint environment.